linux-kernel - Re: [PATCH] mm: remove redundant check in handle_mm

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Date:   Tue, 14 Mar 2023 10:09:04 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Haifeng Xu <haifeng.xu@...pee.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove redundant check in handle_mm_fault

On 14.03.23 09:05, Haifeng Xu wrote:
> 
> 
> On 2023/3/8 17:13, David Hildenbrand wrote:
>> On 08.03.23 10:03, Haifeng Xu wrote:
>>>
>>>
>>> On 2023/3/7 10:48, Matthew Wilcox wrote:
>>>> On Tue, Mar 07, 2023 at 10:36:55AM +0800, Haifeng Xu wrote:
>>>>> On 2023/3/6 21:49, David Hildenbrand wrote:
>>>>>> On 06.03.23 03:49, Haifeng Xu wrote:
>>>>>>> mem_cgroup_oom_synchronize() has checked whether current memcg_in_oom is
>>>>>>> set or not, so remove the check in handle_mm_fault().
>>>>>>
>>>>>> "mem_cgroup_oom_synchronize() will returned immediately if memcg_in_oom is not set, so remove the check from handle_mm_fault()".
>>>>>>
>>>>>> However, that requires now always an indirect function call -- do we care about dropping that optimization?
>>>>>>
>>>>>>
>>>>>
>>>>> If memcg_in_oom is set, we will check it twice, one is from handle_mm_fault(), the other is from mem_cgroup_oom_synchronize(). That seems a bit redundant.
>>>>>
>>>>> if memcg_in_oom is not set, mem_cgroup_oom_synchronize() returns directly. Though it's an indirect function call, but the time spent can be negligible
>>>>> compare to the whole mm user falut preocess. And that won't cause stack overflow error.
>>>>
>>>> I suggest you measure it.
>>>
>>> test steps:
>>> 1) Run command: ./mmap_anon_test(global alloc, so the memcg_in_oom is not set)
>>> 2) Calculate the quotient of cost time and page-fault counts, run 10 rounds and average the results.
>>>
>>> The test result shows that whether using indirect function call or not, the time spent in user fault
>>> is almost the same, about 2.3ms.
>>
>> I guess most of the benchmark time is consumed by allocating fresh pages in your test (also, why exactly do you use MAP_SHARED?).
>>
>> Is 2.3ms the total time for writing to that 1GiB of memory or how did you derive that number? Posting both results would be cleaner (with more digits ;) ).
>>
> 
> Hi Daivd, the details of test result were posted last week. Do you have any suggestions or more concerns about this change?

No, I guess it really doesn't matter performance wise.

One valid question would be: why perform this change at all? The 
redundancy doesn't seem to harm performance either.

If the change would obviously improve code readability it would be easy 
to justify. I'm not convinced, that is the case, but maybe for others.

So FWIW, the change looks good to me and should not affect performance 
in one way or the other. So no objections from my side ...

-- 
Thanks,

David / dhildenb